void main() {
  Person person = Person(name: '张三', age: 20);
  print(person.age);
  print(person.name);
  print('===========================================');
  Teacher teacher = Teacher(Subject: '数学', name: '李四');
}

class Person {
  String name;
  int? age;
  Person({required this.name, this.age = 19});
}

class Teacher extends Person {
  String Subject;
  Teacher({required this.Subject, required super.name, super.age});
}
